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Digestion

Updated Thursday, 27th September 2007

We are what we eat, aren't we? More about food, diet and the importance of healthy eating

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It's a very odd thing,
As odd as can be,
That whatever Miss T eats,
Turns into Miss T.
- Walter De la Mare

Most people would say that the last thing they wanted in their food was chemicals, but, like it or not, you can't avoid them. Everything around you is either a chemical element or compound.

Over 99 per cent of your body weight is made up from as few as 11 of the 90 or so naturally occurring elements! Your body, like the food you eat, is simply a collection of chemicals. Perhaps what people mean is that they want only natural, as opposed to synthetic, chemicals in their food. After all, natural is good and synthetic is bad isn't it? Well, no, it isn't that simple.

Our diet consists of plant and animal materials that often contain many nasty, naturally occurring substances. Even if you ignore all the potentially harmful bacteria in food, many organisms create toxic chemicals as a means of protection. Red kidney beans must be soaked for eight hours and then boiled before use to remove the poisonous chemical lectin. Mouldy peanuts can contain high levels of a natural poison called aflatoxin. Cassava, used to make tapioca, and one of the staple foods of Africa, contains natural chemicals that can produce deadly hydrogen cyanide when eaten. Like kidney beans, cassava has to be treated before it can be safely consumed.

So what's in the food that you eat?

Apart from water, all foods contain just six different types of chemicals: fats, proteins, carbohydrates, minerals, vitamins and fibre. We need these chemicals for three purposes: to control bodily processes, for growth and repair (the word ‘restaurant’ comes from the French word meaning ‘to restore’), and for energy. Most Western diets are well balanced, in that they contain enough vitamins and minerals to ensure that the body functions efficiently. We also consume enough protein and carbohydrate for growth and repair.

We all know that fats (and their liquid equivalent, oils) are slimy water-insoluble materials, but what of the others? Proteins are used as structural materials in many organisms; for example, in hair, nails and tendons. So one source of protein is meat, but one of the richest sources is low-fat soya.

Carbohydrates include chemicals like sugar, starch (also used to stiffen collars!) and the alginates found in seaweed. In fact, dried seaweed is very nutritious and we could easily survive for a long time eating nothing else. Baked beans contain a carbohydrate that’s not easily broken down in the small intestine, but is broken down in the large intestine, leading to flatulence!

The minerals you need in your diet include a number of chemical elements. A deficiency in any of these minerals can cause problems. For instance, if you take in less than 0.0002g of iodine a day, the thyroid gland in your neck can become dangerously enlarged, a condition known as goitre.

Vitamins are a rag-bag of assorted chemicals needed to keep your bodily processes going. They are easily obtained from our diet, and we have evolved so that we no longer need to make them ourselves. Vitamin deficiency can lead to a variety of diseases. For example, too little vitamin D gives rise to rickets, while insufficient vitamin C causes scurvy.

Fibre is mostly cellulose, a carbohydrate used as a structural material in plants. Our bodies can’t digest the fibre in food and it passes straight through the gut.

Look at the nutritional details printed on the packaging of some of the processed foods in your fridge. List the headings under which this information is given. What do you think are the main, nutritional components of the food that we eat?

 

Element
Daily Adult Requirement
Source
Purpose
sodium 4.5g salt maintains fluid-level balance, involved in nerve function
calcium 0.5g dairy products production of bones and teeth
iron 0.01g meat, meat products and bread oxygen transport in the blood
iodine 0.0002g seafood and dairy products formation of thyroid hormones

Food and energy

The energy in food is contained in a variety of molecules, especially fats, carbohydrates and proteins. In general, weight for weight, fats provide about twice as much energy as proteins and carbohydrates, which is why fat-free diets are so effective for slimmers. It’s amazing to think that there’s enough energy in a large slice of buttered toast to keep a cross-country runner going for about ten minutes.

The problem for many people lies in eating the right amount of this amount of food consumed annually by an average family of four ‘energetic’ food. Maintaining your body temperature and just moving around are the main ways in which we use up much of our energy.

If you’re young, exercise regularly and do a strenuous job, you use up a lot of energy and therefore need to eat a large amount of food. If you’re less energetic, you need to eat less. For example, an active 20-year-old male uses up about 14 000 kJ a day, whereas a sedentary 75-year-old female needs half this amount (kJ stands for kilojoules, a unit of energy).

Food labels often contain a statement like ‘100g provides . . . [a certain number of] . . . kJ’. The more energy-rich a food is, the more kJ it contains and the more fattening it is. The trick is to ensure that the number of kJ you burn up roughly matches the number of kJ you take in as food. If it doesn’t, and your kJ input is bigger than your output, then the surplus energy is stored as fat and you put on weight. If you eat less than you need, then you use up body fat and so get thinner.

Where does the energy in food come from?

Strictly speaking, all of the energy in your food derives ultimately from the sun. During photosynthesis, green plants absorb sunlight, carbon dioxide from the air and water and nutrients from the soil, to produce carbohydrates (luckily for us, a by-product of photosynthesis is the oxygen we breathe!).

The energy in sunlight is stored in the chemical bonds of the carbohydrates that make up the plant’s cells; for instance, the wheat grains used in breadmaking are simply chemical energy stores built up by the wheat plant.

What about meat?

Well, all animals ultimately depend on plant life for their existence, so they too can be said to originate from sunlight. Indeed, feeding grain to cattle, which are then fed to humans, is a rather inefficient way of using that grain. Converting the sun’s energy into plant-derived food for human consumption is inefficient enough as it is (at 0.2%), without reducing the efficiency of the process even further by involving other animals.

The chemistry of digestion

When you eat a plant, or the food derived from it, you merely reverse its energy-storage process, breaking down (digesting) the chemicals in the plant or animal, and releasing the energy contained in the chemical bonds. The food is burned with oxygen in the body's cells, producing energy, carbon dioxide (exhaled via the lungs) and water. Digesting food is therefore a bit like burning wood on a fire to generate energy in the form of heat and light.

In the case of food, however, the combustion takes place in a much more controlled way. The rate of combustion is regulated by chemicals in your stomach and intestine called enzymes, that ensure the slow release of the energy.

Most of the chemicals in food consist of large molecules that must be digestion broken down to be useful. Some, like vitamins, minerals, glucose and water are already small enough to pass through the gut wall and don't therefore need to be digested. Those that do are decomposed by enzymes. The breakdown products we want are then absorbed into the blood stream for use elsewhere in the body. The remaining solid material, which is mainly fibre, continues on its journey until it is excreted.

 

Weblinks

Department for Environment, Food and Rural Affairs (DEFRA)
British Nutrition Foundation
The UK Food Directory
International Food Information Council

The BBC and the Open University are not responsible for the content of external websites

Reading

Book 9 of S103, Discovering Science, The Open University

Book 3 of SK220, Human Biology and Health, The Open University

Book 3 of ST240, Our Chemical Environment, The Open University

Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace

 

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